Indexing problem

> I have a table in Informix Online Dynamic Server 7.2 with 6 columns and > 10,000 rows. I created a composite unique index on two columns ( one is > serial with not null and the other integer with not null) and dupls index > on integer column. In a select query Iam comparing serial column with > program variable value and joining with other table using integer column > in the where clause. Query retreival time is 75 seconds . Then I created > an index on serial column . Query retreival time is 2 seconds. My question > is why it is faster in the second case and why can't the composite index > is enough. > > Thanks, > MSK > > > >